CODEWITHSUNDEEP
c++multithreadingmpi
A.N.R.I
A.N.R.I

Reputation: 2051

MPI: How to start three functions which will be executed in different threads

I have 3 function and 4 cores. I want execute each function in new thread using MPI and C++ I write this

int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&size);
size--;
if (rank == 0)
{
    Thread1();
}
else 
{
    if(rank == 1)
    {
        Thread2();
    }
    else
    {
        Thread3();
    }
}
MPI_Finalize();

But it execute just Thread1(). How i must change code?

Thanks!

Upvotes: 2

Views: 1228

Answers (2)

Acorbe
Acorbe

Reputation: 8391

Print to screen the current value of variable size (possibly without decrementing it) and you will find 1. That is: "there is 1 process running".

You are likely running your compiled code the wrong way. Consider to use mpirun (or mpiexec, depending on your MPI implementation) to execute it, i.e.

 mpirun -np 4 ./MyCompiledCode

the -np parameter specifies the number of processes you will start (doing so, your MPI_Comm_size will be 4 as you expect).

Currently, though, you are not using anything explicitly owing to C++. You can consider some C++ binding of MPI such as Boost.MPI.

I worked a little bit on the code you provided. I changed it a little bit producing this working mpi code (I provided some needed correction in capital letters).

FYI:

  • compilation (under gcc, mpich):

     $ mpicxx -c mpi1.cpp 
 $ mpicxx -o mpi1 mpi1.o

  • execution

    $ mpirun -np 4 ./mpi1

  • output

    size is 4
size is 4
size is 4
2 function started.
thread2
3 function started.
thread3
3 function ended.
2 function ended.
size is 4
1 function started.
thread1
1 function ended.

be aware that stdout is likely messed out.

Are you sure you are compiling your code the right way?

Upvotes: 6

Hristo Iliev
Hristo Iliev

Reputation: 74355

You problem is that MPI provides no way to feed console input into many processes but only into process with rank 0. Because of the first three lines in main:

int main(int argc, char *argv[]){

    int oper;
    std::cout << "Enter Size:";
    std::cin >> oper;           // <------- The problem is right here

    Operations* operations = new Operations(oper);
    int rank, size;
    MPI_Init(&argc, &argv);
    int tid;
    MPI_Comm_rank(MPI_COMM_WORLD, &tid);
    switch(tid)
    {

all processes but rank 0 block waiting for console input which they cannot get. You should rewrite the beginning of your main function as follows:

int main(int argc, char *argv[]){

    int oper;

    MPI_Init(&argc, &argv);
    int tid;
    MPI_Comm_rank(MPI_COMM_WORLD, &tid);

    if (tid == 0) {
       std::cout << "Enter Size:";
       std::cin >> oper;
    }
    MPI_Bcast(&oper, 1, MPI_INT, 0, MPI_COMM_WORLD);

    Operations* operations = new Operations(oper);
    switch(tid)
    {

It works as follows: only rank 0 displays the prompt and then reads the console input into oper. Then a broadcast of the value of oper from rank 0 is performed so all other processes obtain the correct value, create the Operations object and then branch to the appropriate function.

Upvotes: 2

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